Structure and formation method of semiconductor device with nanosheet structure

ABSTRACT

A semiconductor device structure and a method for forming a semiconductor device structure are provided. The semiconductor device structure includes a stack of channel structures over a semiconductor fin and a gate stack wrapped around the channel structures. The semiconductor device structure also includes a source/drain epitaxial structure adjacent to the channel structures and multiple inner spacers. Each of the inner spacers is between the gate stack and the source/drain epitaxial structure. The semiconductor device structure further includes an isolation structure between the semiconductor fin and the source/drain epitaxial structure.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs. Each generation has smaller and more complex circuits than the previous generation.

Over the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometric size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling-down process generally provides benefits by increasing production efficiency and lowering associated costs.

However, these advances have increased the complexity of processing and manufacturing ICs. Since feature sizes continue to decrease, fabrication processes continue to become more difficult to perform. Therefore, it is a challenge to form reliable semiconductor devices at smaller and smaller sizes.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It should be noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A-1B are top views of various stages of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments.

FIGS. 2A-2D are cross-sectional views of various stages of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments.

FIGS. 3A-3N are cross-sectional views of various stages of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments.

FIG. 4 is a cross-sectional view of an intermediate stage of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The term “substantially” in the description, such as in “substantially flat” or in “substantially coplanar”, etc., will be understood by the person skilled in the art. In some embodiments the adjective substantially may be removed. Where applicable, the term “substantially” may also include embodiments with “entirely”, “completely”, “all”, etc. Where applicable, the term “substantially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, including 100%. Furthermore, terms such as “substantially parallel” or “substantially perpendicular” are to be interpreted as not to exclude insignificant deviation from the specified arrangement and may include for example deviations of up to 10° in some embodiments. The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y in some embodiments.

Terms such as “about” in conjunction with a specific distance or size are to be interpreted so as not to exclude insignificant deviation from the specified distance or size and may include for example deviations of up to 10% in some embodiments. The term “about” in relation to a numerical value x may mean x±5 or 10% in some embodiments.

Embodiments of the disclosure may relate to FinFET structure having fins. The fins may be patterned using any suitable method. For example, the fins may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in some embodiments, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers may then be used to pattern the fins. However, the fins may be formed using one or more other applicable processes.

Embodiments of the disclosure may relate to the gate all around (GAA) transistor structures. The GAA structure may be patterned using any suitable method. For example, the structures may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. In some embodiments, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in some embodiments, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers may then be used to pattern the GAA structure.

Some embodiments of the disclosure are described. Additional operations can be provided before, during, and/or after the stages described in these embodiments. Some of the stages that are described can be replaced or eliminated for different embodiments. Additional features can be added to the semiconductor device structure. Some of the features described below can be replaced or eliminated for different embodiments. Although some embodiments are discussed with operations performed in a particular order, these operations may be performed in another logical order.

FIGS. 2A-2D are cross-sectional views of various stages of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments. As shown in FIG. 2A, a semiconductor substrate 100 is received or provided. In some embodiments, the semiconductor substrate 100 is a bulk semiconductor substrate, such as a semiconductor wafer. The semiconductor substrate 100 may include silicon or other elementary semiconductor materials such as germanium. The semiconductor substrate 100 may be un-doped or doped (e.g., p-type, n-type, or a combination thereof). In some embodiments, the semiconductor substrate 100 includes an epitaxially grown semiconductor layer on a dielectric layer. The epitaxially grown semiconductor layer may be made of silicon germanium, silicon, germanium, one or more other suitable materials, or a combination thereof.

In some other embodiments, the semiconductor substrate 100 includes a compound semiconductor. For example, the compound semiconductor includes one or more III-V compound semiconductors having a composition defined by the formula Al_(X1)Ga_(X2)In_(X3)As_(Y1)P_(Y2)N_(Y3)Sb_(Y4), where X1, X2, X3, Y1, Y2, Y3, and Y4 represent relative proportions. Each of them is greater than or equal to zero, and added together they equal 1. The compound semiconductor may include silicon carbide, gallium arsenide, indium arsenide, indium phosphide, one or more other suitable compound semiconductors, or a combination thereof. Other suitable substrate including II-VI compound semiconductors may also be used.

In some embodiments, the semiconductor substrate 100 is an active layer of a semiconductor-on-insulator (SOI) substrate. The SOI substrate may be fabricated using a separation by implantation of oxygen (SIMOX) process, a wafer bonding process, another applicable method, or a combination thereof. In some other embodiments, the semiconductor substrate 100 includes a multi-layered structure. For example, the semiconductor substrate 100 includes a silicon-germanium layer formed on a bulk silicon layer.

As shown in FIG. 2A, a semiconductor stack having multiple semiconductor layers is formed over the semiconductor substrate 100, in accordance with some embodiments. In some embodiments, the semiconductor stack includes multiple semiconductor layers 102 a, 102 b, 102 c, and 102 d. The semiconductor stack also includes multiple semiconductor layers 104 a, 104 b, 104 c, and 104 d. In some embodiments, the semiconductor layers 102 a-102 d and the semiconductor layers 104 a-104 d are laid out alternately, as shown in FIG. 2A.

In some embodiments, the semiconductor layers 102 a-102 d function as sacrificial layers that will be removed in a subsequent process to release the semiconductor layers 104 a-104 d. The semiconductor layers 104 a-104 d that are released may function as channel structures of one or more transistors.

In some embodiments, the semiconductor layers 104 a-104 d that will be used to form channel structures are made of a material that is different than that of the semiconductor layers 102 a-102 d. In some embodiments, the semiconductor layers 104 a-104 d are made of or include silicon, germanium, other suitable materials, or a combination thereof. In some embodiments, the semiconductor layers 102 a-102 d are made of or include silicon germanium. In some other embodiments, the semiconductor layers 104 a-104 d are made of silicon germanium, and the semiconductor layers 102 a-102 d are made of silicon germanium with different atomic concentration of germanium than that of the semiconductor layers 104 a-104 s. As a result, different etching selectivity and/or different oxidation rates during subsequent processing may be achieved between the semiconductor layers 102 a-102 d and the semiconductor layers 104 a-104 d.

The present disclosure contemplates that the semiconductor layers 102 a-102 d and the semiconductor layers 104 a-104 d include any combination of semiconductor materials that can provide desired etching selectivity, desired oxidation rate differences, and/or desired performance characteristics (e.g., materials that maximize current flow).

In some embodiments, the semiconductor layers 102 a-102 d and 104 a-104 d are formed using multiple epitaxial growth operations. Each of the semiconductor layers 102 a-102 d and 104 a-104 d may be formed using a selective epitaxial growth (SEG) process, a CVD process (e.g., a vapor-phase epitaxy (VPE) process, a low-pressure chemical vapor deposition (LPCVD) process, and/or an ultra-high vacuum CVD (UHV-CVD) process), a molecular beam epitaxy process, one or more other applicable processes, or a combination thereof.

In some embodiments, the semiconductor layers 102 a-102 d and 104 a-104 d are grown in-situ in the same process chamber. In some embodiments, the growth of the semiconductor layers 102 a-102 d and 104 a-104 d are alternately and sequentially performed in the same process chamber to complete the formation of the semiconductor stack. In some embodiments, the vacuum of the process chamber is not broken before the epitaxial growth of the semiconductor stack is accomplished.

Afterwards, hard mask elements are formed over the semiconductor stack to assist in a subsequent patterning of the semiconductor stack. One or more photolithography processes and one or more etching processes are used to pattern the semiconductor stack into fin structures 106A, 106B, 106C, 106D, and 106E, as shown in FIG. 2B in accordance with some embodiments.

The fin structures 106A-106E may be patterned by any suitable method. For example, the fin structures 106A-106E may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Double-patterning or multi-patterning processes may combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process.

The semiconductor stack is partially removed to form trenches 112, as shown in FIG. 2B. Each of the fin structures 106A-106E may include portions of the semiconductor layers 102 a-102 d and 104 a-104 d and semiconductor fins 101A, 101B, 101C, 101D, and 101E. The semiconductor substrate 100 may also be partially removed during the etching process that forms the fin structures 106A-106E. Protruding portions of the semiconductor substrate 100 that remain form the semiconductor fins 101A-101E.

Each of the hard mask elements may include a first mask layer 108 and a second mask layer 110. The first mask layer 108 and the second mask layer 110 may be made of different materials. In some embodiments, the first mask layer 108 is made of a material that has good adhesion to the semiconductor layer 104 d. The first mask layer 108 may be made of silicon oxide, germanium oxide, silicon germanium oxide, one or more other suitable materials, or a combination thereof. The second layer 110 may be made of silicon nitride, silicon oxynitride, silicon carbide, one or more other suitable materials, or a combination thereof.

FIGS. 1A-1B are top views of various stages of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments. In some embodiments, the fin structures 106A-106E are oriented lengthwise. In some embodiments, the longitudinal extending directions of the fin structures 106A-106E are substantially parallel to each other, as shown in FIG. 1A. In some embodiments, FIG. 2B is a cross-sectional view of the structure taken along the line 2B-2B in FIG. 1A.

As shown in FIG. 2C, an isolation structure 115 is formed to surround lower portions of the fin structures 106A-106E, in accordance with some embodiments. In some embodiments, the isolation structure 115 includes a dielectric filling 114 and a liner layer 113 that is adjacent to the semiconductor fins 101A-101E. In some embodiments, the semiconductor fins 101A-101E protrude from the top surface of the isolation structure 115.

In some embodiments, one or more dielectric layers are deposited over the fin structures 106A-106E and the semiconductor substrate 100 to overfill the trenches 112. The dielectric layers may be made of silicon oxide, silicon oxynitride, borosilicate glass (BSG), phosphoric silicate glass (PSG), borophosphosilicate glass (BPSG), fluorinated silicate glass (FSG), low-k material, porous dielectric material, one or more other suitable materials, or a combination thereof. The liner layer 113 may be made of or include silicon nitride, silicon oxynitride, silicon carbide, silicon oxycarbide, one or more other suitable materials, or a combination thereof. The dielectric layers and the liner layer 113 may be deposited using a flowable chemical vapor deposition (FCVD) process, an atomic layer deposition (ALD) process, a chemical vapor deposition (CVD) process, one or more other applicable processes, or a combination thereof.

Afterwards, a planarization process is used to partially remove the dielectric layers and the liner layer 113. The hard mask elements (including the first mask layer 108 and the second mask layer 110) may also function as stop layers of the planarization process. The planarization process may include a chemical mechanical polishing (CMP) process, a grinding process, a dry polishing process, an etching process, one or more other applicable processes, or a combination thereof.

Afterwards, one or more etching back processes are used to partially remove the dielectric layers and the liner layer 113. As a result, the remaining portion of the dielectric layers forms the dielectric filling 114 of the isolation structure 115. Upper portions of the fin structures 106A-106E protrude from the top surface of the isolation structure 115, as shown in FIG. 2C.

In some embodiments, the etching back process for forming the isolation structure 115 is carefully controlled to ensure that the topmost surface of the isolation structure 115 is positioned at a suitable height level, as shown in FIG. 2C. In some embodiments, the topmost surface of the isolation structure 115 is below the bottommost surface of the semiconductor layer 102 a that functions as a sacrificial layer.

Afterwards, the remaining portions of the hard mask elements (including the first mask layer 108 and the second mask layer 110) are removed. Alternatively, in some other embodiments, the hard mask elements are removed or consumed during the planarization process and/or the etching back process that forms the isolation structure 115.

Afterwards, dummy gate stacks 120A and 120B are formed to extend across the fin structures 106A-106E, as shown in FIG. 1B in accordance with some embodiments. In some embodiments, FIG. 2D is a cross-sectional view of the structure taken along the line 2D-2D in FIG. 1B. FIGS. 3A-3K are cross-sectional views of various stages of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments. In some embodiments, FIG. 3A is a cross-sectional view of the structure taken along the line 3A-3A in FIG. 1B.

As shown in FIGS. 1B, 2D, and 3A, the dummy gate stacks 120A and 120B are formed to partially cover and to extend across the fin structures 106A-106E, in accordance with some embodiments. In some embodiments, the dummy gate stacks 120A and 120B are wrapped around portions of the fin structures 106A-106E. As shown in FIG. 2D, the dummy gate stack 120B extends across and is wrapped around the fin structures 106A-106E. As shown in FIG. 1B, other portions of the fin structures 106A-106E are exposed without being covered by the dummy gate stack 120A or 120B.

As shown in FIGS. 2D and 3A, each of the dummy gate stacks 120A and 120B includes a dummy gate dielectric layer 116 and a dummy gate electrode 118. The dummy gate dielectric layer 116 may be made of or include silicon oxide or another suitable material. The dummy gate electrodes 118 may be made of or include polysilicon or another suitable material.

In some embodiments, a dummy gate dielectric material layer and a dummy gate electrode layer are sequentially deposited over the isolation structure 115 and the fin structures 106A-106E. The dummy gate dielectric material layer may be deposited using an ALD process, a CVD process, one or more other applicable processes, or a combination thereof. The dummy gate electrode layer may be deposited using a CVD process. Afterwards, the dummy gate dielectric material layer and the dummy gate electrode layer are patterned to form the dummy gate stacks 120A and 120B.

In some embodiments, hard mask elements including mask layers 122 and 124 are used to assist in the patterning process for forming the dummy gate stacks 120A and 120B. With the hard mask elements as an etching mask, one or more etching processes are used to partially remove the dummy gate dielectric material layer and the dummy gate electrode layer. As a result, remaining portions of the dummy gate dielectric material layer and the dummy gate electrode layer form the dummy gate stacks 120A and 120B that include the dummy gate dielectric layer 116 and the dummy gate electrodes 118.

As shown in FIG. 3B, spacer layers 126 and 128 are afterwards deposited over the dummy gate stacks 120A and 120B, the fin structures 106A-106E, and the isolation structures 115, in accordance with some embodiments. The spacer layers 126 and 128 extend along the tops and sidewalls of the dummy gate stacks 120A and 120B, as shown in FIG. 3B.

The spacer layers 126 and 128 are made of different materials. The spacer layer 126 may be made of a dielectric material that has a low dielectric constant. The spacer layer 126 may be made of or include silicon carbide, silicon oxycarbide, carbon-containing silicon oxynitride, silicon oxide, one or more other suitable materials, or a combination thereof. In some embodiments, the spacer layer 126 is a single layer. In some other embodiments, the spacer layer 126 includes multiple sub-layers. Some of the sub-layers may be made of different materials. Some of the sub-layers may be made of similar materials with different compositions. For example, one of the sub-layers may have a greater atomic concentration of carbon than other sub-layers.

The spacer layer 128 may be made of a dielectric material that can provide more protection to the gate stacks during subsequent processes. The spacer layer 128 may have a greater dielectric constant than that of the spacer layer 126. The spacer layer 128 may be made of silicon nitride, silicon oxynitride, carbon-containing silicon nitride, carbon-containing silicon oxynitride, one or more other suitable materials, or a combination thereof. The spacer layers 126 and 128 may be sequentially deposited using a CVD process, an ALD process, a physical vapor deposition (PVD) process, one or more other applicable processes, or a combination thereof.

As shown in FIG. 3C, the spacer layers 126 and 128 are partially removed, in accordance with some embodiments. One or more anisotropic etching processes may be used to partially remove the spacer layers 126 and 128. As a result, remaining portions of the spacer layers 126 and 128 form spacer elements 126′ and 128′, respectively. The spacer elements 126′ and 128′ extend along the sidewalls of the dummy gate stacks 120A and 120B, as shown in FIG. 3C.

Afterwards, with the dummy gate stacks 120A and 120B and the spacer elements 126′ and 128′ as an etching mask, the fin structures 106A-106E are partially removed. As shown in FIG. 3C, the fin structure 106C is partially removed to form recesses 130, in accordance with some embodiments. The recesses 130 may be used to contain epitaxial structures (such as source/drain structures) that will be formed later. One or more etching processes may be used to form the recesses 130.

In some embodiments, a dry etching process is used to form the recesses 130. Alternatively, a wet etching process may be used to form the recesses 130. In some embodiments, each of the recesses 130 penetrates into the fin structure 106C. In some embodiments, the recesses 130 further extend into the semiconductor fin 101C, as shown in FIG. 3C. In some embodiments, the spacer elements 126′ and 128′ and the recesses 130 are simultaneously formed using the same etching process.

In some embodiments, each of the recesses 130 has slanted sidewalls. Upper portions of the recesses 130 are larger (or wider) than lower portions of the recesses 130. In these cases, due to the profile of the recesses 130, an upper semiconductor layer (such as the semiconductor layer 104 d) is shorter than a lower semiconductor layer (such as the semiconductor layer 104 b).

However, embodiments of the disclosure have many variations. Process conditions may be varied to modify the profile of the recesses 130. In some other embodiments, the recesses 130 have substantially vertical sidewalls. In these cases, due to the profile of the recesses 130, an upper semiconductor layer (such as the semiconductor layer 104 d) is substantially as wide as a lower semiconductor layer (such as the semiconductor layer 104 b).

As shown in FIG. 3D, a protective layer 302 is deposited over the sidewalls and bottom of the recesses 130, in accordance with some embodiments. In some embodiments, the protective layer 302 further extends along the top and sidewalls of the dummy gate stacks 120A and 120B. In some embodiments, the protective layer 302 extends conformally along the sidewalls of the recesses 130 and the dummy gate stacks 120A and 120B. The protective layer 302 may protect the fin structure 106C and help to maintain the profile of the upper portions of the recesses 130 during a subsequent etching process.

The protective layer 302 may be made of an oxide material, a nitride material, one or more other suitable materials, or a combination thereof. In some embodiments, the protective layer 302 is made of silicon oxide. The protective layer 302 may be deposited using an ALD process, a CVD process, one or more other applicable processes, or a combination thereof.

In some embodiments, the reaction gas mixture used for forming the protective layer 302 includes a silicon-containing gas, a halogen-containing gas, an oxygen-containing gas, one or more other suitable gases, or a combination thereof. In some embodiments, the reaction gas mixture includes SiCl₄, HBr, and O₂.

In some embodiments, the introducing of the reaction gas mixture includes multiple steps. For example, in a first reaction cycle, a reaction gas mixture including SiCl₄ and HBr is introduced into a reaction chamber. Afterwards, in a second reaction cycle, O₂ is introduced into the reaction chamber.

As shown in FIG. 3E, the semiconductor fin 101C is partially removed from the bottoms of the recesses 130, in accordance with some embodiments. As a result, each of the recesses 130 extends further downward in the semiconductor fin 101C, as shown in FIG. 3E. In some embodiments, lower portions 304 of the recesses 130 extend laterally in the semiconductor fin 101C. The bowing profile of the lower portions 304 may help to a subsequent formation of isolation structures. The lower portion 304 of the recess 130 may have a width that is in a range from about 20 nm to about 60 nm.

In some embodiments, the lower portions 304 of the recesses 130 are formed using an etching process. The protective layer 302 may prevent the fin structure 106C from being etched during the formation of the lower portions 304 of the recesses 130. In some embodiments, the lower portions 304 of the recesses 130 are formed using a dry etching process. The reaction gas mixture used in the dry etching process may include HBr, NF₃, He, one or more other suitable gases, or a combination thereof.

In some embodiments, an anisotropic etching operation is performed to remove the portions of the protective layer 302 positioned at the bottom portions of the recesses 130, exposing the semiconductor fin 101C. Afterwards, an isotropic etching operation is used to etch the exposed portions of the semiconductor fin 101C. As a result, the recesses 130 extend further downward and laterally in the semiconductor fin 101C. The lower portions 304 of the recesses 130 are formed. The formation of the protective layer 302 and/or the lower portions 304 of the recesses 130 may be performed at an operation temperature that is in a range from about 50 degrees C. to about 70 degrees C.

As shown in FIG. 3F, the remaining portions of the protective layer 302 are removed, in accordance with some embodiments. In some embodiments, a wet clean operation is used to remove the protective layer 302. For example, a dilute HF solution may be used to remove the protective layer 302.

As shown in FIG. 3G, the semiconductor layers 102 a-102 d are laterally etched, in accordance with some embodiments. The semiconductor layers 102 a-102 d that are exposed by the recess 130 are thus recessed. As a result, edges of the semiconductor layers 102 a-102 d retreat from edges of the semiconductor layers 104 a-104 d. As shown in FIG. 3G, recesses 132 are formed due to the lateral etching of the semiconductor layers 102 a-102 d. The recesses 132 may be used to contain inner spacers that will be formed later.

The semiconductor layers 102 a-102 d may be laterally etched using a wet etching process, a dry etching process, or a combination thereof. In some other embodiments, the semiconductor layers 102 a-102 d are partially oxidized before being laterally etched.

During the lateral etching of the semiconductor layers 102 a-102 d, the semiconductor layers 104 a-104 d may also be slightly etched. As a result, edge portions of the semiconductor layers 104 a-104 d are partially etched and thus shrink to become edge elements 105 a-105 d, as shown in FIG. 3G. As shown in FIG. 3G, each of the edge elements 105 a-105 d of the semiconductor layers 104 a-104 d is thinner than the corresponding inner portion of the semiconductor layers 104 a-104 d.

As shown in FIG. 3H, an insulating layer 134 is deposited over the structure shown in FIG. 3G, in accordance with some embodiments. The insulating layer 134 covers the dummy gate stacks 120A and 120B and the sidewalls and bottom of the recess 130. As shown in FIG. 3H, the insulating layer 134 fills the recesses 132 and extends along the sidewalls of the lower portions 302 of the recesses 132.

The insulating layer 134 may be made of or include carbon-containing silicon nitride (SiCN), carbon-containing silicon oxynitride (SiOCN), carbon-containing silicon oxide (SiOC), silicon oxide, silicon nitride, one or more other suitable materials, or a combination thereof. In some embodiments, the insulating layer 134 is a single layer. In some other embodiments, the insulating layer 134 includes multiple sub-layers. Some of the sub-layers may be made of different materials and/or contain different compositions. The insulating layer 134 may be deposited using a CVD process, an ALD process, one or more other applicable processes, or a combination thereof.

As shown in FIG. 3I, the insulating layer 134 is partially removed, in accordance with some embodiments. The remaining portions of the insulating layer 134 form inner spacers 136 and isolation structures 306, as shown in FIG. 3I. In some embodiments, the isolation structures 306 are positioned between the semiconductor substrate 100 and the inner spacers 136.

FIG. 4 is a cross-sectional view of an intermediate stage of a process for forming a portion of a semiconductor device structure, in accordance with some embodiments. In some embodiments, FIG. 4 is an enlarged cross-sectional view of a portion of the structure shown in FIG. 3I.

As shown in FIG. 4 , the inner spacer 136 has a first length L₁ measured in a vertical direction and a second length L₂ measured in a horizontal direction. In some embodiments, the first length L₁ is in a range from about 5 nm to about 9 nm. In some embodiments, the second length L₂ is in a range from about 4 nm to about 8 nm. In some other embodiments, the inner spacer 136 has a substantially vertical sidewall and thus has a longer second length L₂. The second length L₂ may be in a range from about 6 nm to about 12 nm.

As shown in FIG. 4 , the isolation structure 306 has a height H. In some embodiments, the height H is in a range from about 5 nm to about 30 nm. In some embodiments, the isolation structure 306 is taller than the inner spacer 136. In these cases, the height H is in a range from about 6 nm to about 30 nm. However, embodiments of the disclosure are not limited thereto. In some other embodiments, the inner spacer 136 is taller than the isolation structure 306. Alternatively, in some other embodiments, the height H of the isolation structure 306 is substantially equal to the first length L₁ of the inner spacer 136.

In some embodiments, the isolation structure 306 has a first sidewall and a second sidewall that meet at a widest part of the isolation structure 306, as shown in FIG. 4 . The widest part of the isolation structure 306 has a widest width W. The widest width W may be in a range from about 2 nm to about 9 nm. As shown in FIG. 4 , the first sidewall and the second sidewall of the isolation structure 306 form an angle θ. In some embodiments, the angle θ is in a range from about 120 degrees to about 150 degrees.

In some embodiments, the upper portion of the isolation structure 306 shrinks from the widest part with the width W to the top of the isolation structure 306, as shown in FIG. 4 . In some embodiments, the lower portion of the isolation structure 306 shrinks from the widest part to the bottom of the isolation structure 306.

As shown in FIG. 4 , the widest part of the isolation structure 306 has a height h. The height h may be in a range from about 3 nm to about 25 nm.

In some embodiments, the insulating layer 134 is partially removed using an etching process. The etching process may include a dry etching process, a wet etching process, or a combination thereof. In some embodiments, the inner spacers 136 and the isolation structures 306 are made of the same material since they are formed from portions of the same layer (i.e., the insulating layer 134). The compositions of the inner spacers 136 and the isolation structures 306 are substantially the same. The isolation structures 306 may have a first dielectric constant, and the inner spacers 136 may have a second dielectric constant. In some embodiments, the first dielectric constant is substantially equal to the second dielectric constant.

The inner spacers 136 cover the edges of the semiconductor layers 102 a-102 d. The inner spacers 136 may be used to prevent subsequently formed epitaxial structures (that function as, for example, source/drain structures) from being damaged during a subsequent removing process of the sacrificial layers 102 b-102 d. In some embodiments, the inner spacers 136 are made of a low-k material that has a lower dielectric constant than that of silicon oxide. In these cases, the inner spacers 136 may also be used to reduce parasitic capacitance between the subsequently formed source/drain structures and the gate stacks. As a result, the operation speed of the semiconductor device structure may be improved.

The isolation structures 306 extend the sidewalls of the lower portions 302 of the recesses 130. The isolation structures 306 may be used to prevent or reduce current leakage between subsequently formed epitaxial structures (that function as, for example, source/drain structures). As a result, the operation speed of the semiconductor device structure may be improved.

In some embodiments, after the etching process for forming the inner spacers 136 and the isolation structures 306, portions of the semiconductor fin 101C originally covered by the insulating layer 134 are exposed by the recesses 130, as shown in FIG. 3I. The edges of the semiconductor layers 104 a-104 d are also exposed by the recesses 130, as shown in FIG. 3I.

As shown in FIG. 3J, epitaxial structures 138 are formed, in accordance with some embodiments. In some embodiments, the epitaxial structures 138 fill the recesses 130 including the lower portions 302, as shown in FIG. 3J. In some other embodiments, the epitaxial structures 138 overfill the recesses 130. In these cases, the top surfaces of the epitaxial structures 138 may be higher than the top surface of the dummy gate dielectric layer 116. In some other embodiments, the epitaxial structures 138 partially fill the recesses 130. In some embodiments, the epitaxial structures 138 are formed to be in direct contact with the semiconductor fin 101C. In some embodiments, the isolation structures 306 are in direct contact with the epitaxial structures 138 and the semiconductor fin 101C.

As shown in FIG. 3J, the isolation structures 306 extend along the sidewalls of the lower portions 302 of the recesses 130. Each of the isolation structures 306 has a first side and a second side that is opposite to the first side. In some embodiments, the first side is adjacent to the semiconductor fin 101C, and the second side is adjacent to the epitaxial structure 138, as shown in FIG. 3J. As shown in FIG. 3J, the isolation structures 306 extend along opposite sidewalls of the lower portion of the epitaxial structure 138. The isolation structures 306 may thus be used to significantly prevent or reduce current leakage between the nearby epitaxial structures 138. As a result, the operation speed of the semiconductor device structure may be improved.

In some embodiments, the epitaxial structures 138 connect to the semiconductor layers 104 a-104 d. Each of the semiconductor layers 104 a-104 d is sandwiched between the epitaxial structures 138. In some embodiments, the epitaxial structures 138 are p-type doped regions. The epitaxial structures 138 may include epitaxially grown silicon germanium (SiGe), epitaxially grown silicon, or another suitable epitaxially grown semiconductor material.

However, embodiments of the disclosure are not limited thereto. In some other embodiments, the epitaxial structures 138 are n-type doped regions. The epitaxial structures 138 may include epitaxially grown silicon, epitaxially grown silicon carbide (SiC), epitaxially grown germanium, or another suitable epitaxially grown semiconductor material.

In some embodiments, the epitaxial structures 138 are formed using a selective epitaxial growth (SEG) process, a CVD process (e.g., a vapor-phase epitaxy (VPE) process, a low-pressure chemical vapor deposition (LPCVD) process, and/or an ultra-high vacuum CVD (UHV-CVD) process), a molecular beam epitaxy process, one or more other applicable processes, or a combination thereof. In some embodiments, the formation of the epitaxial structures 138 involves one or more etching processes that are used to fine-tune the shapes of the epitaxial structures 138.

In some embodiments, the epitaxial structures 138 are doped with the same type of dopant. In some embodiments, the epitaxial structures 138 are doped with the same dopant. In some embodiments, the epitaxial structures 138 are doped with one or more suitable p-type dopants. For example, the epitaxial structures 138 are SiGe source/drain features or Si source/drain features that are doped with boron (B), gallium (Ga), indium (In), or another suitable dopant. In some other embodiments, the epitaxial structures 138 are doped with one or more suitable n-type dopants. For example, the epitaxial structures 138 are Si source/drain features doped with phosphor (P), antimony (Sb), or another suitable dopant.

In some embodiments, the epitaxial structures 138 are doped in-situ during their epitaxial growth. The initial reaction gas mixture for forming the epitaxial structures 138 contains dopants. In some other embodiments, the epitaxial structures 138 are not doped during the growth of the epitaxial structures 138. Instead, after the formation of the epitaxial structures 138, the epitaxial structures 138 are doped in a subsequent process.

In some embodiments, the doping is achieved by using an ion implantation process, a plasma immersion ion implantation process, a gas and/or solid source diffusion process, one or more other applicable processes, or a combination thereof. In some embodiments, the epitaxial structures 138 are further exposed to one or more annealing processes to activate the dopants. For example, a rapid thermal annealing process is used.

As shown in FIG. 3K, a contact etch stop layer 139 and a dielectric layer 140 are formed to cover the epitaxial structures 138 and to surround the dummy gate stacks 120A and 120B, in accordance with some embodiments. The contact etch stop layer 139 may be made of or include silicon nitride, silicon oxynitride, silicon carbide, aluminum oxide, one or more other suitable materials, or a combination thereof. The dielectric layer 140 may be made of or include silicon oxide, silicon oxynitride, borosilicate glass (BSG), phosphoric silicate glass (PSG), borophosphosilicate glass (BPSG), fluorinated silicate glass (FSG), low-k material, porous dielectric material, one or more other suitable materials, or a combination thereof.

In some embodiments, an etch stop material layer and a dielectric material layer are sequentially deposited over the structure shown in FIG. 3J. The etch stop material layer may be deposited using a CVD process, an ALD process, a PVD process, one or more other applicable processes, or a combination thereof. The dielectric material layer may be deposited using an FCVD process, a CVD process, an ALD process, one or more other applicable processes, or a combination thereof.

Afterwards, a planarization process is used to partially remove the etch stop material layer and the dielectric material layer. As a result, the remaining portions of the etch stop material layer and the dielectric material layer respectively form the contact etch stop layer 139 and the dielectric layer 140, as shown in FIG. 3K. The planarization process may include a CMP process, a grinding process, an etching process, a dry polishing process, one or more other applicable processes, or a combination thereof.

In some embodiments, the mask layers 122 and 124 are removed during the planarization process. In some embodiments, after the planarization process, the top surfaces of the contact etch stop layer 139, the dielectric layer 140, and the dummy gate electrodes 118 are substantially level with each other.

As shown in FIG. 3L, the dummy gate electrodes 118 are removed to form trenches 142 using one or more etching processes, in accordance with some embodiments. The trenches 142 are surrounded by the dielectric layer 140. The trenches 142 expose the dummy gate dielectric layer 116.

As shown in FIG. 3M, the dummy gate dielectric layer 116 and the semiconductor layers 102 a-102 d (that function as sacrificial layers) are removed, in accordance with some embodiments. In some embodiments, one or more etching processes are used to remove the dummy gate dielectric layer 116 and the semiconductor layers 102 a-102 d. As a result, recesses 144 are formed, as shown in FIG. 3M.

Due to high etching selectivity, the semiconductor layers 104 a-104 d are slightly (or substantially not) etched. The remaining portions of the semiconductor layers 104 a-104 d form multiple semiconductor nanostructures 104 a′-104 d′. The semiconductor nanostructures 104 a′-104 d′ are constructed by or made up of the remaining portions of the semiconductor layers 104 a-104 d. The semiconductor nanostructures 104 a′-104 d′ suspended over the semiconductor fin 101C may function as channel structures of transistors. In some embodiments, the epitaxial structures 138 are formed to be in direct contact with the semiconductor fin 101C and the semiconductor nanostructures 104 a′-104 d′.

In some embodiments, the etchant used for removing the semiconductor layers 102 a-102 d also slightly removes the semiconductor layers 104 a-104 d that form the semiconductor nanostructures 104 a′-104 d′. As a result, the obtained semiconductor nanostructures 104 a′-104 d′ become thinner after the removal of the semiconductor layers 102 a-102 d. In some embodiments, each of the semiconductor nanostructures 104 a′-104 d′ is thinner than the edge portions 105 a-105 d since the edge portions 105 a-105 d are surrounded by other elements and thus are prevented from being reached and etched by the etchant.

After the removal of the semiconductor layers 102 a-102 d (that function as sacrificial layers), the recesses 144 are formed. The recesses 144 connect to the trench 142 and surround each of the semiconductor nanostructures 104 a′-104 d′. As shown in FIG. 3M, even if the recesses 144 between the semiconductor nanostructures 104 a′-104 d′ are formed, the semiconductor nanostructures 104 a′-104 d′ remain being held by the epitaxial structures 138. Therefore, after the removal of the semiconductor layers 102 a-102 d (that function as sacrificial layers), the released semiconductor nanostructures 104 a′-104 d′ are prevented from falling down.

During the removal of the semiconductor layers 102 a-102 d (that function as sacrificial layers), the inner spacers 136 protect the epitaxial structures 138 from being etched or damaged. The quality and reliability of the semiconductor device structure are improved.

In some embodiments, the inner spacers 136 are in direct contact with the edge portions 105 a-105 d of the semiconductor nanostructures 104 a′-104 d′, as shown in FIG. 3M. In some embodiments, the isolation structures 306 are separated from the semiconductor nanostructures 104 a′-104 d′.

As shown in FIG. 3N, metal gate stacks 156A and 156B are formed to fill the trenches 142, in accordance with some embodiments. The metal gate stacks 156A and 156B further extend into the recesses 144 to wrap around each of the semiconductor nanostructures 104 a′-104 d′.

Each of the metal gate stacks 156A and 156B includes multiple metal gate stack layers. Each of the metal gate stacks 156A and 156B may include a gate dielectric layer 150 and a metal gate electrode 152. The metal gate electrode 152 may include a work function layer. The metal gate electrode 152 may further include a conductive filling. In some embodiments, the formation of the metal gate stacks 156A and 156B involves the deposition of multiple metal gate stack layers over the dielectric layer 140 to fill the trenches 142 and the recesses 144. The metal gate stack layers extend into the recesses 144 to wrap around each of the semiconductor nanostructures 104 a′-104 d′.

In some embodiments, the gate dielectric layer 150 is made of or includes a dielectric material with high dielectric constant (high-K). The gate dielectric layer 150 may be made of or include hafnium oxide, zirconium oxide, aluminum oxide, hafnium dioxide-alumina alloy, hafnium silicon oxide, hafnium silicon oxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafnium zirconium oxide, one or more other suitable high-K materials, or a combination thereof. The gate dielectric layer 150 may be deposited using an ALD process, a CVD process, one or more other applicable processes, or a combination thereof.

In some embodiments, before the formation of the gate dielectric layer 150, an interfacial layers are formed on the surfaces of the semiconductor nanostructures 104 a′-104 d′. The interfacial layers are very thin and are made of, for example, silicon oxide or germanium oxide. In some embodiments, the interfacial layers are formed by applying an oxidizing agent on the surfaces of the semiconductor nanostructures 104 a′-104 d′. For example, a hydrogen peroxide-containing liquid may be applied or provided on the surfaces of the semiconductor nanostructures 104 a′-104 d′ so as to form the interfacial layers.

The work function layer of the metal gate electrode 152 may be used to provide the desired work function for transistors to enhance device performance including improved threshold voltage. In some embodiments, the work function layer is used for forming a PMOS device. The work function layer is a p-type work function layer. The p-type work function layer is capable of providing a work function value suitable for the device, such as equal to or greater than about 4.8 eV.

The p-type work function layer may include metal, metal carbide, metal nitride, other suitable materials, or a combination thereof. For example, the p-type metal includes tantalum nitride, tungsten nitride, titanium, titanium nitride, one or more other suitable materials, or a combination thereof.

In some other embodiments, the work function layer is used for forming an NMOS device. The work function layer is an n-type work function layer. The n-type work function layer is capable of providing a work function value suitable for the device, such as equal to or less than about 4.5 eV.

The n-type work function layer may include metal, metal carbide, metal nitride, or a combination thereof. For example, the n-type work function layer includes titanium nitride, tantalum, tantalum nitride, one or more other suitable materials, or a combination thereof. In some embodiments, the n-type work function is an aluminum-containing layer. The aluminum-containing layer may be made of or include TiAlC, TiAlO, TiAlN, one or more other suitable materials, or a combination thereof.

The work function layer may also be made of or include hafnium, zirconium, titanium, tantalum, aluminum, metal carbides (e.g., hafnium carbide, zirconium carbide, titanium carbide, aluminum carbide), aluminides, ruthenium, palladium, platinum, cobalt, nickel, conductive metal oxides, or a combinations thereof. The thickness and/or the compositions of the work function layer may be fine-tuned to adjust the work function level.

The work function layer may be deposited over the gate dielectric layer 150 using an ALD process, a CVD process, a PVD process, an electroplating process, an electroless plating process, one or more other applicable processes, or a combination thereof.

In some embodiments, a barrier layer is formed before the work function layer to interface the gate dielectric layer 150 with the subsequently formed work function layer. The barrier layer may also be used to prevent diffusion between the gate dielectric layer 150 and the subsequently formed work function layer. The barrier layer may be made of or include a metal-containing material. The metal-containing material may include titanium nitride, tantalum nitride, one or more other suitable materials, or a combination thereof. The barrier layer may be deposited using an ALD process, a CVD process, a PVD process, an electroplating process, an electroless plating process, one or more other applicable processes, or a combination thereof.

In some embodiments, the conductive fillings of the metal gate electrodes 152 are made of or include a metal material. The metal material may include tungsten, aluminum, copper, cobalt, one or more other suitable materials, or a combination thereof. A conductive layer used for forming the conductive filling may be deposited over the work function layer using a CVD process, an ALD process, a PVD process, an electroplating process, an electroless plating process, a spin coating process, one or more other applicable processes, or a combination thereof.

In some embodiments, a blocking layer is formed over the work function layer before the formation of the conductive layer used for forming the conductive filling. The blocking layer may be used to prevent the subsequently formed conductive layer from diffusing or penetrating into the work function layer. The blocking layer may be made of or include tantalum nitride, titanium nitride, one or more other suitable materials, or a combination thereof. The blocking layer may be deposited using an ALD process, a PVD process, an electroplating process, an electroless plating process, one or more other applicable processes, or a combination thereof.

Afterwards, a planarization process is performed to remove the portions of the metal gate stack layers outside of the trenches 142, in accordance with some embodiments. As a result, the remaining portions of the metal gate stack layers form the metal gate stacks 156A and 156B, as shown in FIG. 3N.

In some embodiments, the conductive filling does not extend into the recesses 144 since the recesses 144 are small and have been filled with other elements such as the gate dielectric layer 150 and the work function layer. However, embodiments of the disclosure are not limited thereto. In some other embodiments, a portion of the conductive filling extends into the recesses 144, especially for the lower recesses 144 that may have larger space.

In some embodiments, there are four channel structures (such as the semiconductor nanostructures 104 a′-104 d′) formed. However, embodiments of the disclosure are not limited thereto. Many variations and/or modifications can be made to embodiments of the disclosure. In some embodiments, the total number of semiconductor nanostructures is greater than four. In some other embodiments, the total number of semiconductor nanostructures is smaller than four. The total number of semiconductor nanostructures (or channel structures) of each semiconductor device structure may be fine-tuned to meet requirements. For example, the total number of semiconductor nanostructures may be 3 to 8. The semiconductor nanostructures may have many applicable profiles. The semiconductor nanostructures may include nanosheets, nanowires, or other suitable nanostructures.

Embodiments of the disclosure form a semiconductor device structure with an epitaxial structures and an isolation structure. The isolation structure extends along the sidewall of the lower portion of the epitaxial structure. The isolation structure may significantly prevent or reduce current leakage between the nearby epitaxial structures. The performance and reliability of the semiconductor device structure may therefore be improved.

In accordance with some embodiments, a semiconductor device structure is provided. The semiconductor device structure includes a stack of channel structures over a semiconductor fin and a gate stack wrapped around the channel structures. The semiconductor device structure also includes a source/drain epitaxial structure adjacent to the channel structures and multiple inner spacers. Each of the inner spacers is between the gate stack and the source/drain epitaxial structure. The semiconductor device structure further includes an isolation structure between the semiconductor fin and the source/drain epitaxial structure.

In accordance with some embodiments, a semiconductor device structure is provided. The semiconductor device structure includes a stack of nanosheets over a semiconductor fin and a gate stack wrapped around the nanosheets. The semiconductor device structure also includes an epitaxial structure penetrating into the semiconductor fin, and the epitaxial structure connects the nanosheets. The semiconductor device structure further includes an isolation structure having a first side and a second side, and the first side is opposite to the second side. The first side is adjacent to the semiconductor fin, and the second side is adjacent to the epitaxial structure.

In accordance with some embodiments, a method for forming a semiconductor device structure is provided. The method includes forming a fin structure over a semiconductor substrate. The fin structure has a semiconductor fin, multiple sacrificial layers and multiple semiconductor layers. The sacrificial layers and the semiconductor layers are laid out alternately. The method also includes partially removing the fin structure to form a recess. The method further includes partially removing the semiconductor fin from a bottom of the recess so that the recess extends further downward and laterally in the semiconductor fin. In addition, the method includes forming an epitaxial structure in the recess and removing the dummy gate stack. The method includes removing the sacrificial layers to form multiple semiconductor nanostructures made of remaining portions of the semiconductor layers. The method also includes forming a metal gate stack to wrap around the semiconductor nanostructures.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor device structure, comprising: a stack of channel structures over a semiconductor fin; a gate stack wrapped around the channel structures; a source/drain epitaxial structure adjacent to the channel structures; a plurality of inner spacers, wherein each of the inner spacers is between the gate stack and the source/drain epitaxial structure; and an isolation structure between the semiconductor fin and the source/drain epitaxial structure, wherein the isolation structure has a first sidewall and a second sidewall, the first sidewall and the second sidewall meet at a widest part of the isolation structure, and a lower portion of the isolation structure shrinks from the widest part to a bottom of the isolation structure.
 2. The semiconductor device structure as claimed in claim 1, wherein compositions of the isolation structure and the inner spacers are substantially the same.
 3. The semiconductor device structure as claimed in claim 1, wherein the isolation structure is between the semiconductor substrate and the inner spacers.
 4. The semiconductor device structure as claimed in claim 1, wherein the isolation structure is in direct contact with the source/drain epitaxial structure and the semiconductor fin.
 5. The semiconductor device structure as claimed in claim 4, wherein the source/drain epitaxial structure is in direct contact with the semiconductor fin.
 6. The semiconductor device structure as claimed in claim 1, wherein the isolation structure is taller than each of the inner spacers.
 7. The semiconductor device structure as claimed in claim 1, wherein the first sidewall and the second sidewall form an angle, and the angle is in a range from about 120 degrees to about 150 degrees.
 8. The semiconductor device structure as claimed in claim 1, wherein an upper portion of the isolation structure shrinks from the widest part to a top of the isolation structure.
 9. The semiconductor device structure as claimed in claim 1, wherein a bottommost end of the epitaxial structure is in direct contact with the semiconductor fin.
 10. The semiconductor device structure as claimed in claim 1, wherein the epitaxial structure is closer to a bottom of the semiconductor fin than the isolation structure.
 11. A semiconductor device structure, comprising: a stack of nanosheets over a substrate; a gate stack wrapped around the nanosheets; an epitaxial structure connecting the nanosheets; and an isolation structure having a first side and a second side, wherein the second side is between the first side and the epitaxial structure, and the epitaxial structure is closer to a bottom of the substrate than the isolation structure.
 12. The semiconductor device structure as claimed in claim 11, wherein the isolation structure is separated from the nanosheets.
 13. The semiconductor device structure as claimed in claim 11, further comprising a second isolation structure, wherein the isolation structure extends along a first sidewall of a lower portion of the epitaxial structure, the second isolation structure extends along a second sidewall of the lower portion of the epitaxial structure, and the first sidewall is opposite to the second sidewall.
 14. The semiconductor device structure as claimed in claim 13, wherein the isolation structure and the second isolation structure are separated from each other by the epitaxial structure.
 15. The semiconductor device structure as claimed in claim 11, further comprising a first inner spacer and a second inner spacer, wherein the second inner spacer is between the first inner spacer and the isolation structure, the second inner spacer is between two of the nanosheets, and the second inner spacer is between the epitaxial structure and the gate stack.
 16. The semiconductor device structure as claimed in claim 15, wherein the isolation structure has a first dielectric constant, the first inner spacer and the second inner spacer have a second dielectric constant, and the first dielectric constant is substantially equal to the second dielectric constant.
 17. A method for forming a semiconductor device structure, comprising: forming a fin structure over a semiconductor substrate, wherein the fin structure has a semiconductor fin, a plurality of sacrificial layers and a plurality of semiconductor layers, wherein the sacrificial layers and the semiconductor layers are laid out alternately; forming a dummy gate stack to cover a portion of the fin structure; partially removing the fin structure to form a recess; partially removing the semiconductor fin from a bottom of the recess so that the recess extends further downward and laterally in the semiconductor fin; forming an isolation structure over a sidewall of a lower portion of the recess in the semiconductor fin; forming a protective layer over sidewalls of the recess before the semiconductor fin is partially removed; removing a remaining portion of the protective layer after the semiconductor fin is partially removed and before the isolation structure is formed; forming an epitaxial structure in the recess; removing the dummy gate stack; removing the sacrificial layers to form a plurality of semiconductor nanostructures made of remaining portions of the semiconductor layers; and forming a metal gate stack to wrap around the semiconductor nanostructures.
 18. The method for forming a semiconductor device structure as claimed in claim 17, further comprising: recessing the sacrificial layers exposed by the recess; and forming inner spacers to cover edges of the sacrificial layers before the epitaxial structure is formed.
 19. The method for forming a semiconductor device structure as claimed in claim 18, further comprising: forming an insulating layer to cover the sidewalls and a bottom of the recess before the epitaxial structure is formed; and partially removing the insulating layer, wherein remaining portions of the insulating layer form the isolation structure and the inner spacers.
 20. The method for forming a semiconductor device structure as claimed in claim 17, wherein the epitaxial structure is formed to be in direct contact with the semiconductor fin and the semiconductor nanostructures. 